Hi AndreasRabo,
I have the same problem here. With the picture of your post 3840x2160 it is working but it seems like it is wrong size. When I download it has 1920x1080.
For my own test I was trying with:
1824x1368 → Working
4608x2184 → Not working
Did you found a solution for this so fare?
Found this now: New Screenly ose can't handle high resolution photos